Redox reaction is generally utilized for measuring the concentration of a particular component in a body fluid, such as the concentration of glucose in blood. On the other hand, portable hand-held blood glucose level measuring apparatuses are available so that diabetics can easily measure the blood glucose level at home or away from home. For measuring the blood glucose level using such a portable blood glucose level measuring apparatus, a disposable biosensor for providing an enzyme reaction system is mounted to the blood glucose level measuring apparatus, and blood is supplied to the biosensor. Recently, to reduce the burden on the diabetics, a blood glucose level measuring apparatus integrated with a lancet has been developed so that lancing of the skin with a lancet and introduction of blood from the skin to the biosensor can be performed in immediate succession.
Various biosensors have been put to practical use, and an example of such biosensors is shown in FIGS. 23 and 24A. The illustrated biosensor 9 comprises a substrate 93 having an upper surface formed with a counterpart electrode 90, an operative electrode 91 and a reference electrode 92 each in the form of a strip, and a cover 95 laminated on the substrate via a pair of spacers 94. The substrate 93, the spacers 94 and the cover 95 define a capillary 96. The capillary 96 includes end openings 96a, 96b, and a reagent layer 97 in a solid state extending transversely over the counterpart electrode 90, the operative electrode 91 and the reference electrode 92. The reagent layer 97 contains oxidoreductase and an electron carrier. The cover 95 has an inner surface provided with a hydrophilic layer 99 formed by applying a surface-active agent such as lecithin. The hydrophilic layer 99 is provided to cause highly viscous blood (high Hct value) to properly move through the capillary 96.
In the biosensor 9, blood 98 is introduced through one opening 96a, as shown in 24A. The blood 98 travels through the capillary 96 toward the other opening 96b by capillary action under the assisting action of hydrophilic layer 99 while dissolving the reagent layer 97. In the reagent layer 97, oxidoreductase oxidizes glucose in the blood while reducing the electron carrier. When a potential is applied across the operative electrode 91 and the counterpart electrode 90, the electron carrier is oxidized (releases electrons). The blood glucose level measuring apparatus measures an oxidation current. The glucose concentration is figured out based on the oxidation current.
However, due to the provision of the openings 96a and 96b at the opposite ends of the capillary 96, the reagent layer 97 may be exposed to water entering the capillary 96 through the openings 96a and 96b if such water exists around the biosensor 9 during the storage thereof. Specifically, even when the blood 98 is not introduced into the capillary 96, water reduces the electron carrier. Therefore, not only the electrons released due to the reaction with glucose but also the water-induced electrons are detected as oxidation current. Such background current (noise) by the electrons due to water results in measurement errors. Moreover, since the solubility of the reagent layer 97 changes in accordance with the hygroscopicity, the time taken for filling the capillary with blood (suction time) cannot be kept constant, which causes difficulty in proper measurement.
Since the hydrophilic layer 99 is provided by applying a surface-active agent dissolved in an organic solvent to the cover 95 and then drying, the surface-active agent is easily removed from the cover. When the surface-active agent is removed from the cover 95, the region from which the surface-active agent has been removed has a low wettability, which causes a low travel speed. Further, the surface-active agent thus removed may move through the capillary 96 together with the blood 98 (particularly blood cells). While the hydrophilic surface-active agent is likely to move through a hydrophobic portion, the spacers 94 are made of a hydrophobic double-sided tape or fixed to the substrate 93 and the cover 95 with a hydrophobic double-sided tape. Therefore, as shown in FIG. 24A, the blood moves faster along the spacers 94 than along the inner surface of the cover 95. Thus, variation is caused in the speed distribution of the blood flow in the capillary 96.
When such a phenomenon occurs, spreading of the blood entirely over the reagent layer 97 takes a long time. Therefore, when oxidation current is to be measured at a predetermined time after the blood introduction, the reagent layer 97 may not have been completely dissolved at the time of the measurement. Further, as shown in FIG. 24B, the blood flowing along the spacers 94 may sometimes reach the opening 96b earlier than blood flowing along other portions and may close the opening 96b to hinder the movement of the blood 98, thereby producing an air stagnating portion 96A in the capillary 96. When the air stagnating portion 96A is produced, the reagent layer 97 at the air stagnating portion 96A is not dissolved. In this way, variation in the speed distribution of the blood flow hinders sufficient dissolving of the reagent layer 97 within a predetermined time, which may increase the measurement errors and deteriorate the reproducibility.
When a blood glucose level measuring apparatus which is not provided with a lancet is used, a lancet disclosed in JP-A-9-266898, for example, may be used for introducing blood to the biosensor 9. In using the lancet, it is necessary to lance the skin with the lancet and bring the blood from the skin into contact with the opening 96a of the biosensor 9.
At that time, the user needs to perform the operation carefully while visually confirming the contact of blood with the opening 96a. Further, in the biosensor 9, blood need be introduced after it is confirmed that the amount of blood needed for the measurement is extracted from the skin, because otherwise, the amount of blood introduced to the biosensor 9 may be insufficient or the time taken for filling the capillary with the blood after the starting of the blood introduction is not kept constant.
In this way, the use of the biosensor 9 for a blood glucose level measuring apparatus which is not provided with a lancet involves a large burden on the user. Since the burden on the eyes is particularly large, the use is very difficult for a person who has weak eyesight due to the progress of diabetes, and the measurement accuracy is likely to be deteriorated.
In the case where the biosensor 9 is used for a blood glucose level measuring apparatus provided with a lancet, the arrangement for automatically bringing the blood from the skin into contact with the small opening 96a cannot be realized due to various technical difficulties in terms of the positioning of the biosensor 9 and the timing of contacting, for example.